Telescopic sights, also called riflescopes, enable soldiers and riflemen to aim at distant targets with high accuracy. In telescopic sights, the “point of aim” is often designated by a reticle or cross hairs. The eyepiece of a telescopic sight has an “exit pupil”. This is a virtual pupil located on the eyepiece optical axis at a set distance behind the eyepiece. This distance is called “eye relief”. To see the whole field of view, the shooter must position his head such that his eye pupil is at the exit pupil. In a riflscope, the eye relief must be least 5 cm to prevent the eyepiece hitting the shooter's forehead when the rifle recoils. On high-power hunting rifles, a longer eye relief such as 9 cm or 12 cm is preferred. The concepts of exit pupil and eye relief are well-known in the art and are described in most riflescope manufacturers catalogs.
When a riflescope is mounted on a rifle, its optical axis will be positioned higher than the optical axis of the rifle's open sights. Therefore, to aim through his riflescope, the shooter has to raise his head from the position he would otherwise uses if he were aiming with the open sights. This situation is illustrated in FIG. 1(a). The required shift in head position is more pronounced when the riflescope has a large objective lens which necessitates a high mount. A high riflescope mount is also required on certain bolt-action rifles (e.g. the famous Mauser M-98) to allow for the bolt to operate freely. To help the shooter align his eye with the optical axis of a high-mounted riflescope, some rifle stocks are equipped with a raised or adjustable comb (see FIG. 1(a)). However, a raised comb would make the rifle unsuitable for use with open sights. A better solution would be to design the riflescope with tilted viewing axis such that its exit pupil is lower than its optical axis. This way, the shooter can see the target image without the need to raise his head.
Another application where a tilted viewing axis is advantageous is when a secondary sight such as a non-magnifying reflector sight is mounted on top of the main telescopic sight (See FIG. 1(b)). This configuration is often used by soldiers in combat situations where a soldier needs to engage enemy positions at both close and far distances in quick succession. He can use the non-magnifying reflector sight for engaging targets at close distances. The telescopic sight is used for aiming at distant targets.
A major drawback of the configuration shown in FIG. 1(b) is that the telescopic sight and the reflector sight require separate eye positions. What is needed is a telescopic sight with tilted viewing axis such that the soldier need not change his eye position when switching his gaze from the reflector sight to the telescoping sight and vice versa.